A hot-rolled steel sheet produced in a comparatively inexpensive way is widely used for various industrial equipment including an automobile and others. Since in recent years, from the viewpoint of regulating an amount of emission of carbon dioxide in order to take measures against global warming, an improvement in the fuel efficiency of an automobile has been required, a high strength hot-rolled steel sheet has been widely applied to the automobile so as to reduce the weight of a vehicle body and to secure a crashworthiness. Further, here recently, a high strength hot-dip galvannealed hot-rolled steel sheet for which a high strength hot-rolled steel sheet is a base steel sheet has been demanded for a chassis part such as a suspension arm which requires especially a corrosion resistance.
In a steel sheet adopted as an automobile part, not only strength but also various working properties, which are required when a part is formed, such as press formability and weldability also need to be satisfied. As to the press forming of the chassis part, a stretch flanging and a barring are extremely frequently used and hence the high strength hot-dip galvannealed hot-rolled steel sheet supplied for producing the chassis part is required to have an excellent hole expansibility.
Further, there is a case where the high strength steel sheet is applied for a part of which a high crashworthiness is required and for a part which needs to avoid plastic deformation when a large load is applied thereto. In such case, the high strength steel sheet is required to have a high yield ratio. Hence, there is a case where also the high strength hot-dip galvannealed hot-rolled steel sheet is required to have the high yield ratio.
In general, in the high strength hot-dip galvannealed hot-rolled steel sheet, in order to satisfy both of the high yield ratio and the excellent hole expansibility, a steel structure of a single phase structure tends to have ferrite, bainitic ferrite, or bainite as a main phase, and finely precipitate carbide of Ti, Nb, V or the like and Cu to thereby uniformly strengthen the main phase. For examples, the high strength hot-dip galvannealed hot-rolled steel sheets have been developed as shown below.
In a Patent Document 1 discloses a high strength hot-dip galvannealed hot-rolled sheet which has a steel structure mainly comprising bainite and which has the contents of not only Ti, Nb, V but also P, Cu, Cr, Mo, Ni suitably controlled, thereby being improved in a fatigue resistance property of a welded portion under a corrosive environment. However, this steel sheet needs to have a large amount of expensive alloying elements such as Cu, Ni, Mo added thereto and hence is not suitable for a mass production from the perspective of economy. Further, the steel sheet may have the slightly poor hole expansibility.
Patent Document 2 discloses a high strength hot-dip galvannealed hot-rolled steel sheet in which a hot-rolled steel sheet having a (ferrite+bainite) structure is subjected to a thermal history of an optimum hot-dip galvannealing process to suitably control a structure, the shape of carbide, and solid solution C, thereby being improved in the hole expansibility. However, in more than 650 MPa of a tensile strength of a product made of this steel sheet is, the product cannot have a sufficient hole expansibility.
Document 3 discloses a hot-dip galvanized hot-rolled steel sheet in which a steel structure substantially comprising a single phase of ferrite has Ti carbide finely dispersed, the Ti carbide containing Mo and/or W. However, this steel sheet needs to have extremely expensive alloying element such as Mo and W added thereto and hence is not suitable for the mass production from the perspective of economy.
Document 4 discloses a hot-dip galvanized hot-rolled steel sheet in which a structure mainly comprising ferrite and having a dispersion state of pearlite and cementite optimally controlled has Nb, V, Ti added thereto to increase precipitation strengthening, thereby being improved in the hole expansibility. However, in at least 650 MPa of a high strength of the steel sheet, it is possible that the steel sheet does not have a sufficient hole expansibility.